Ancient Warfare

For much
of human existence, it appears that organized warfare did not happen,
because people lived in small groups far apart, and hardly ever got
together in big enough numbers for a war.

But as soon as people started farming, they had to fight wars. When you decide to get most of your food from farming instead of hunting and gathering, you've made a commitment to the land where you planted your seeds. You have to keep that land safe until you harvest the food, or your whole group will starve. So people fought to defend their land. The more men you have, the better you can fight. So people began to fight to get more land, so they could feed more people, so they could defend their land. Men did the fighting, and women stayed home to make and cook for more and more babies, so they could grow up and fight too. Not everybody liked this strategy, but the people who fought and made a lot of babies won, and the other people lost and became their slaves, or their serfs.

It's easy to go to a protest and say that war is bad,
but a look at some of the wars that were fought during antiquity will
show that people generally believe they have good reasons for fighting
wars. Hardly anybody thinks killing people is a good idea, unless they
can see no other solution for some serious problems of their own.

Africans marching to war, 2000 BC

One reason that people fight wars is that there is
no longer enough food where they are,
and they have to find more land and food somewhere, or they and their
children will starve. This is probably the cause of all those invasions
from Siberia south into Iran and China:
the Persians, the Parthians,
the Huns, the
Turks, the Mongols.
It is also the cause of the Germans
invading the Roman Empire.

Prisoners of War (Romans, 200 AD)

Another reason that people fight wars is that they
are afraid of being invaded, and they believe that they can be safer
by conquering dangerous enemies on their borders, than by waiting to
be attacked. This is probably the cause of the Persian
Wars, and the Peloponnesian
War, and the expansion of the Roman Empire.

A third reason why people fight wars is that they believe
God wants them to convert people to their religion. This is at least
one of the reasons for the Arab
invasion of the Roman Empire and the Parthian
Empire, and it is part of the reason for the Crusades.

Or people fight wars because they have been fighting
them, and it is very hard to stop once you get started. You get started
fighting people in order to protect your own boundaries, or because
you have been invaded, but then suppose you win? There you are with
all these soldiers, trained killers with spears and swords and armor.
It's hard to tell them you're going to stop paying them now, and please
go home quietly. They're liable to take that badly. Much safer to keep
on fighting. This is behind the conquests of Alexander
the Great, for instance, and maybe the Assyrians
(More about victorious armies).